I pushed 4 boats with a 6hp 4-stroke from Sand Wash to Jack Creek (maybe 4 or 5 hours of actual run time) on about a gallon, maybe one and a quarter tops. I kept it at ~3/4 throttle most of the time because wide open made it louder but didn't seem any faster. We were going about 5 mph including the current.
Second merritrd's suggestion of making sure you're registered, it's not worth the fine. Don't try to do it online - go to a state park with the serial number from the raft so that they can actually give you the carbon copy that is your temporary tag. My official numbers didn't show up until almost the end of the 60-day temporary window.